In the previous chapter, we saw some of the techniques used to probe and monitor electrical and other signals from the human body and from single cells. Our understanding of the basic mechanisms behind physiological phenomena is aided by advances, particularly in the last half century, in knowledge of the structure and synthesis of the basic molecular building blocks of the cell. Electron microscopy has allowed the visualization of basic cellular components such as mitochondria, chromosomes, and the cell membrane, but the practical limit of resolution (around 1 nm) prevents the determination of the structure of individual molecules (although this can be inferred from molecular formulas and from knowledge of which parts of the molecule are happy to be close to water (termed hydrophilic) and those that do not (termed hydrophobic). More recently, atomic force microscopy (discussed further in Chapter 27) has been used to visualize the atomic details at membrane surfaces. However, the most important technique used to determine molecular structure has been X-ray crystallography. Because the wavelengths of X-rays (0.01 nm-10 nm) can be less than interatomic distances, details at this scale can be inferred. The particular molecule to be studied has ‹rst to be extracted from the cell and then crystallized, which is, in itself, no mean feat. In crystalline form, the molecule is in a regular lattice, and any repeating feature will give rise to characteristic “spots” in the crystallogram. The basic principles of crystallography are summarized in Figure 4.1. This relies on the work of the Nobel Prize-winning father-and-son team of W. and W. L. Bragg.